A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen or an alcohol, such as methanol or ethanol, is supplied to the anode and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In a hydrogen-fuelled or alcohol-fuelled proton exchange membrane fuel cell (PEMFC), the electrolyte is a solid polymeric membrane, which is electronically insulating and proton conducting. Protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water. The most widely used alcohol fuel is methanol, and this variant of the PEMFC is often referred to as a direct methanol fuel cell (DMFC).
The principal component of the PEMFC is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymeric ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrocatalytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Conventionally, the MEA can be constructed by a number of methods outlined hereinafter:                (i) The electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed either side of an ion-conducting membrane and laminated together to form the five-layer MEA;        (ii) The electrocatalyst layer may be applied to both faces of the ion-conducting membrane to form a catalyst-coated ion-conducting membrane. Subsequently, gas diffusion layers are applied to both faces of the catalyst-coated ion-conducting membrane.        (iii) An MEA can be formed from an ion-conducting membrane coated on one side with an electrocatalyst layer, a gas diffusion layer adjacent to that electrocatalyst layer, and a gas diffusion electrode on the other side of the ion-conducting membrane.        
Conventionally, the MEA is constructed so that the central polymeric ion-conducting membrane extends to the edge of the MEA, with the gas diffusion layers and electrocatalyst layers being smaller in area than the membrane such that there is an area around the periphery of the MEA which comprises ion-conducting membrane only. The area where no electrocatalyst is present is a non-electrochemically active region. Film layers, typically formed from non-ion conducting polymers, are generally positioned around the edge region of the MEA on the exposed surfaces of the ion-conducting membrane where no electrocatalyst is present to seal and/or reinforce the edge of the MEA. An adhesive layer may be present on one or both surfaces of the seal film layer. The seal film layers are conventionally produced by cutting openings in a complete film to form a so-called seal “window frame” that is then positioned around the edge of the MEA on the exposed ion-conducting membrane surfaces.
The layers in the MEA are typically bonded by a lamination process. It is common practice that the polymeric ion-conducting membrane also comprises a reinforcement material, such as a planar porous material, embedded within the thickness of the membrane, to provide for improved mechanical strength of the membrane and thus increased durability of the MEA and lifetime of the fuel cell.
To enable a faster rate of commercialisation of fuel cells and a greater market penetration it is necessary to make further improvements to the MEA design and manufacturing processes to significantly reduce manufacturing costs and increase the manufacturing output rate for the MEA. As such continuous high volume manufacturing processes, wherein a continuous roll of precursor MEA is produced at high speed, are being introduced as alternatives to manufacturing processes where individual MEAs are assembled from the separate singular MEA components.
Typically, much of the polymeric ion-conducting material used in the membrane extends beyond the electrochemically active region into a non-electrochemically active region, often by up to several centimeters. In low geometric area MEAs this non-electrochemically active region can contribute to as much as 50% of the entire MEA geometric area. The membrane which extends beyond the electrochemically active area does not contribute to the activity and performance. The polymeric ion-conducting membrane is one of the most costly components in the fuel cell, and it is thus desirable to minimise its usage. This design approach is commonly practiced for both discrete singular MEA assembly processes as well as for continuous higher speed assembly processes. In the latter case the seal window frame film is provided in a continuous fashion as a so-called seal “ladder” film. The seal film material can also be of high cost, and thus there is a need also to minimise the usage of the seal material in the development of improved MEA manufacturing processes.
WO2015/145127 discloses an approach to the formation of a membrane-seal assembly (MSA) within the MEA which seeks to reduce production costs. An MSA can be considered as a core building block for the manufacture of an MEA, in that it involves the early formation of the membrane component combined with the edge seal films layers. The method involves the formation of the MSA by deposition of an ion-conducting component and a seal component together with a planar reinforcing material and provides a process for these components to be deposited directly only in the regions in which they are required for functionality in the fuel cell. The method ensures that both the components fill the pores in the planar reinforcing material and that there are regions of ion-conducting component completely bounded by seal component across the plane of the MSA produced. As a result, the preferred MSA produced according to WO2015/145127 has central portions of ion-conducting material surrounded with window frames of seal material. Advantageously, this reduces the amount of costly ion-conducting material required. Advantageously, the deposition technique also avoids wastage of the seal material associated with conventional MEA designs. The method described is particularly applicable to a continuous high speed manufacturing process.
Therefore, one aim is to provide an improved process that tackles the drawbacks associated with the conventional prior art and which also provides for a further improved process over that described in WO2015/145127, or at least provides a commercial alternative thereto.
The present invention therefore seeks to provide an improved process for manufacturing a reinforced membrane-seal assembly, which provides for a high utilisation of the membrane material and the seal film material in the reinforced membrane-seal assembly.